The present invention relates to an internal combustion engine provided with a turbocharger having a variable geometry turbine and with a decompression-type braking system.
Internal combustion engines are known, for example from DE-C-197 27 140, provided with a turbocharger having a variable geometry turbine and with a decompression-type braking system.
Decompression-type braking systems are based on the principle of dissipating the compression energy produced inside the engine cylinders to generate braking power; for example, this may be achieved by opening exhaust valves of the engine cylinders at the end of the compression stroke. The efficiency of this solution is increased by the supercharging provided by the turbocharger, which induces an increase in the compression power and, therefore, in braking power.
However, it is known that the efficiency of a turbine decreases as the flow rate of gases decreases, which depends on the angular velocity of the drive shaft (for brevity designated xe2x80x9cengine speedxe2x80x9d in the following); therefore, when the engine braking is actuated at low engine speed, as may happen for example when travelling downhill, the braking effect which can be obtained by decompression is unsatisfactory since the level of the induction pressure of the engine is low and the compression work dissipated is consequently modest.
With a view to improving the efficiency of the engine braking at low rpm, the use of turbochargers has been introduced, which are provided with a variable geometry turbine (VGT), i.e. comprising a rotor and an annular inlet nozzle in which is accommodated a bladed stator adapted to control the effective flow section of the nozzle or by varying the orientation of the blades, or by relative axial sliding between a flow cut-off element forming part of the stator and the nozzle itself.
The effective flow section is regulated as a function of the operating conditions of the engine; in particular, at low engine speeds the stator is maintained in a position of minimum effective flow section, thus bringing about an increase in the velocity of the gases impinging on the rotor. The speed of rotation of the turbine rotor (for brevity designated xe2x80x9cturbine speedxe2x80x9d in the following), which brings about an increase in the supercharging pressure.
At high engine speeds, high flow rates of exhaust gas pass through the turbine; with a view to maintaining the speed of the turbine at a safe level, i.e. at a level such that the turbocharger is not excessively loaded, and so as to contain the thermal stress of the engine and of the turbocharger, it is necessary to increase gradually the effective flow section of the nozzle so as to reduce the velocity at which said gases impinge on the rotor, and therefore to contain the speed of the turbine and the supercharging pressure.
According to DE-C-197 27 140, the effective flow section of the turbine is controlled by maintaining between preset limits a magnitude which is characteristic of the setting of the turbine defined as TBF=ATxc2x7DT/VH, wherein AT is the effective inlet flow section of the turbine, i.e. the controlled variable, DT is the inlet diameter of the turbine rotor and VH is the engine capacity. In particular, this magnitude is maintained below the value 0.005 and, preferably, is between 0.001 and 0.003.
The aforementioned control logic has the drawback of taking into account only geometrical factors of the engine (capacity) and of the turbine (inlet diameter) but not the operating conditions of the engine.
The result of this is that under certain operating conditions, and in particular close to the maximum permitted engine speed, the control which can be achieved with the above-described logic may lead to undesirable values of operating parameters of the engine and of the turbocharger in relation to which there are structural limitations.
It is the object of the present invention to devise an internal combustion engine provided with a turbocharger having a variable geometry turbine and with a decompression-type braking system, which makes it possible to eliminate the above-mentioned drawbacks.
This object is achieved by the present invention in that it relates to an internal combustion engine comprising:
a decompression-type braking device,
a turbocharger provided with a variable geometry turbine having a rotor and an annular nozzle for directing the exhaust gases towards said rotor in which is disposed a bladed stator defining a variable effective flow section of said nozzle,
an actuator for controlling the setting of said stator, and
a control unit for controlling said actuator in response to an actuating signal for the decompression braking and as a function at least of the speed of said engine, so as to increase gradually said effective flow section from a minimum value maintained in a lower portion of the range of variation of the engine speed to a limit value under conditions of maximum braking power,
characterised in that, with reference to said maximum braking power condition, said limit value is equal to at least one minimum safety value (ST) defined by the equation                                           T            0                    ·                      P            i                    ·                      N                          T              ⁢                              xe2x80x83                            ⁢              max                                                            T            i                    ·                      PME            max                    ·                      N            max                              ·                                    S            T                    ·                      D            T                          V              =    K    ,
wherein:
T0 (K) is the ambient temperature,
Ti (K) is the temperature of the gases at the turbine
inlet under said maximum braking power condition,
pi is the pressure of the gases at the turbine inlet under said maximum braking power condition,
PME is a maximum design value of the mean effective pressure of the engine,
NTmax is a maximum permitted value of the turbine speed,
Nmax is a maximum engine speed,
DT is an inlet diameter of the turbine rotor, and
V is the engine capacity,
K is a non-dimensional constant between 0.0175 and 0.0230.